The present invention relates to a method for bending glass sheets, in which method a cold, non-bent glass sheet is placed on a mould, the mould and the glass sheet supported thereby are carried on a mould wagon through preheating and heating station, the glass sheet supported by the mould is heated in each preheating and heating station until the glass reaches a bending temperature, the heated glass sheet supported by the mould is carried to a bending station and the glass sheet is further heated in the bending station so as to effect a desired bending partially or entirely through the action of gravity.
The invention relates also to a bending furnace for glass sheets, comprising a number of successive preheating and heating stations, a bending station, a number of successive cooling stations below said preheating and heating stations, moulds for supporting glass sheets during the course of heating, bending and cooling, and wagons for carrying the moulds from one station to another.
The preferred embodiment of the present invention relates to a method for bending laminated glass sheets, wherein superposedly laminated, non-bent glass sheets are placed on a ring mould, the ring mould and the pair of laminated glass sheets supported thereby are carried on a mould wagon through preheating sections, the pair of glass sheets supported by the mould is transferred into a bending section, the pair of glass sheets is heated to a bending temperature and a bending operation is effected.
The above type of method and furnace apparatus for bending glass sheets are prior known from the commonly-assigned patent publications U.S. Pat. No. 4,497,645 and U.S. Pat. No. 4,986,842. This type of furnace is especially intended for bending laminated windshields gravitationally by means of a ring mould. Naturally, press bending can also be used to assist gravitational bending. When bending typical windshield profiles, heating resistances are used to make sure that the glass sheet has a heat distribution which contributes to the formation of a desired bending shape. For example, due to the steeper bending curves formed therein, the end sections are subjected to more radiation heat than the mid-section which remains relatively flat.
It is prior known from patent publications U.S. Pat. No. 5,009,691 and U.S. Pat. No. 4,441,907 to employ forced convection in a heating step preceding the bending. Neither of these publications says anything about forced convection at the time a glass sheet is already in the mould where it is heated and bent. It is prior known from patent publication U.S. Pat. No. 2,967,378 to employ the local application of radiation heat to a glass sheet to be bent in a mould. From patent publication U.S. Pat. No. 4,755,204 it is prior known to minimize the temperature difference between the mold and the glass by directing hot air around the mold beneath the glass.
The increase of production capacity is one of the basic objectives in further development of the bending lines in vehicle windshields and backlights.
In addition, there has developed a need to produce also such bending shapes, wherein the mid-section of a glass sheet bends to a progressive or uniformly extending curve without the mid-section remaining any flatter than the rest of a glass sheet. Such a shape is preferred e.g. for the reason that the windshield wipers operate more efficiently as their pressure on the glass surface remains more uniform. The aerodynamic reasons can also be a basis for preferring a progressive bending curve. This is not possible to achieve by means of resistance fields used for bending conventional windshield configurations. In principle, it would be possible to alter the resistance field configuration and the disposition of resistances in a manner that one and the same apparatus can be used for bending both conventional windshield shapes and shapes with the mid-section bent to a progressive curve. However, the re-arrangement of resistance fields for this purpose is an inconvenient and expensive operation. The shape control becomes more difficult as the pocket to which the entire glass sheet should be bent becomes deeper and the surface area of a glass sheet becomes larger.
In general, the sag bending takes a long time in order that also the bending shape is under control.
An object of the invention is to provide a method and a furnace assembly of the above type which, are also capable of substantially improving production capacity and effecting intensified bending of the mid-section of a glass sheet in view of producing various bending shapes without having to resort to the structural re-arrangement of resistance fields.
This object is achieved by means of a method of the invention in a manner that, during the course of heating effected in a preheating station and/or in the prebending station and/or in the bending station, the heating of a glass sheet is intensified by the application of convection blast, and during the course of bending the distribution of radiation heat is adjusted.
In a preferred embodiment the blasted air is colder than the surrounding air in the furnace. As a result of this, there is no excessive heating at the blasting point, but the convection effect is spread along the glass surface such that it may be evenly divided or even stronger at the areas surrounding the blasting point.
The convection blast can be focused to any section at which the bending to form a pocket should be intensified. In a typical case, the convection blast is focused on the mid-section of a glass sheet for bending it to a progressive curve.
Convection blast can be maintained throughout the period that the glass is stationary in a preheating station (or in a subsequent prebending or bending station), allowing for the use of a very weak blast which does not locally overheat the glass surface so as to form a "heat lens".
Though convection blast can also be used in a prebending station preceding the bending station and/or in the bending station itself, it is most advantageous to employ convection blast just in preheating stations for bending laminated glass sheets (windshields). The reason for this appears from the following.
The invention is particularly suitable for use in such a method, wherein the bending operation of laminated glass sheets is followed by transferring the mould wagon and the pair of bent glass sheets from an upper track onto a lower track and by cooling the pair of bent glass sheets on the lower track below said preheating sections.
This prior known method involves the following problem. Inside preheating sections develops a temperature difference of about 100.degree. C. between the bottom and top glass. This temperature difference does not equalize even in the bending section. A result of this is that the bottom glass "resists" bending of the top glass and decelerates bending as it is necessary to wait for the bottom glass to heat up to a bending temperature. This effect is particularly pronounced in the process of creating a progressive sag in the middle of a glass piece or in complex bends in the corners.
In view of the above, it is a further object of the invention to overcome this problem and to equalize said temperature difference in a manner such that heating and bending can be accelerated and thus the production capacity of a furnace can be increased while facilitating the attainment of desired bending shapes.
This object is achieved by intensifying the transfer of heat by means of convection to the bottom glass of a pair of glass sheets lying in preheating section by blasting a small amount of air to the bottom surface of said bottom glass for thus reducing a temperature difference between the glass sheets included in a pair of glass sheets lying in preheating section.
In this particular type of furnace the transfer of heat to the bottom glass can be intensified by intensifying the transfer of heat from a glass piece cooling down on the lower track to the bottom glass piece lying on the upper track. In a particular application of the invention, this is effected by blowing a small amount of air through the open floor of a mould wagon from pipes fitted between the upper and lower tracks.
The best mode of the invention offers the following benefits:
the top and bottom glass pieces are brought to an equal temperature prior to the commencement of actual bending by making effective use of a temperature difference (about 100.degree. C.) between the glass pieces lying on lower track and on upper track for heating the bottom glass piece on the upper track by applying convection without separate heating. PA1 The cooling process does not develop spreading or reverse bends in the glasses as a result of temperature differences between top and bottom glasses. PA1 The glass finds its way better to a shape determined by a ring mould, especially in complex bends. PA1 The bending of a progressive sag can be successfully effected with a short bending time.
The homothermal glass facilitates rapid bending of the glass since there is no need in a bending process to wait for the bottom glass to heat up to a bending temperature.
A furnace or lehr assembly of the invention is characterized in that the preheating station and/or prebending station and/or bending station is provided with a convection-air blast pipe extended through the thermally insulated ceiling or wall and having its blasting orifice opening below and/or above a glass sheet placed in the preheating station and/or prebending station and/or bending station for blasting air colder than the surrounding furnace air towards the glass surface from said blasting orifice.
The convection-air blast pipe can be fitted with a pyrometer for measuring the glass temperature. Thus, the blasting action e.g. termination of the blasting, can be controlled on the basis of a temperature measurement effected by the pyrometer. In the first instance, the glass temperature measurement by the pyrometer is used for controlling and adjusting the distribution of radiation heat in the bending station, as disclosed in more detail in publication EP-0486952.